Sarges_heroes2003

My Feedback: (6)

I have been brain storming a way to get a engine for a aircraft like the harrier, the engine needs to make compressed air to shoot out of nozzles but expensive "jet engines" are out of the question because the model engines are not designed to work in a harier. So I would need a compact aircompressor unit that had a high cfm. then it came to me, the most compact type of air compressor I can think of is a centrifucal type super charger. Its small, no reciprocating parts, and puts out high speed compressed air. what if a took a small one of these, spun by a electric motor, with a gear transmition, and "piped" the air to the nozzles. as long as the bends are less than 90 degrees, the uniformity of the flow could be maintained and thus its speed and efficiency.
I wonder if its been done before?? any ideas??

BMatthews

You'd have to do some testing on it to find out if it would support the mass of air to be moved.

A decade or more ago a fellow in Ottawa named Eric Dainty took on the challenge of a fully functional Harrier using a ducted fan setup to provide the thrust. His unit used a single bifurcted pair of nozzles, rather than 2 pairs, located right at the CG of the model with an air bleed to the tail and wingtips for hover control very similar to the full sized Harrier. He did manage to hover and I seem to remember he achieved transistions to level and back to hover. But as I recall he had a lot of trouble achieving an effective airflow through the ducts and nozzles without too much loss.

It would come down to engine power as well as compressor efficiency. From there it would be a case of relative efficiencies between the axial ducted fan method and the centrifigal vane type compressor. I'd guess that the axial produces more velocity in the flow whereas the centrifugal can produce more pressure at a lower flow speed based on the large circumferential area which through good nozzle design may be able to produce the velocity required.

dicknadine

sounds good. however there a lot of questions to be answered. what size plane ? what is the aircraft main propulsion method ? what is the air compressor power method? are the jets manuervable ? etc, etc. dick

Sarges_heroes2003

My Feedback: (6)

I think that a Strong electric motor might do the job of powering the compresor but it would deffinantly need a gear box so it could push enough rpm's. typically a car centifugal (such as vortex mustang superchargers) use a gear box and a pulley run off of the serpentien belt. those run at around 10k rpms and typicaly make good boost at the mustang engines upper rpms. so u have a minimum rpms before u get the advertised boost. cfm might not be what a duct fan but oodels more psi.
for a 5-6 lb. model id say the 10 psi range would be a good place to start. it would need solenoids to control how much goes where. and I wonder if there would be a need for a 5th nozle located under the cockpit to keep it up. don't know why I haven't seen a nozle located there on the real thing. could it posibly have such balance that a 5th nozle was not needed??
a solenoid could do the job on the aft "puffer" port. plus some kind of selector will be needed, probably slewed to the rudder servo for simplicity.

Sarges_heroes2003

My Feedback: (6)

but of course a strong, reliable glow engine could work as well. might need some kind of pull rope to start it but thats ok.

BMatthews

Jets are basically a reaction device. Someone can correct me on this later but I believe you'll find that it all comes down to how much mass can you push back at what speed to produce a forward thrust. As such PSI doesn't mean much but CFM and jet stream velocity does. However it's lucky that by using a constrictive duct design you can fairly easily convert PSI to velocity for the same CFM flow through the system.

Gearing will certainly be needed since you're probably going to be using a model turbojet sized centrifugal compressor hooked to that motor and to provide the flow required for a typical jet they need to spin up around 100K rpms. Also bear in mind that you'll only be using the flow of the compressor so do not expect any turbojet type performance without spinning the compressor even faster by quite a bit. You see, you won't be adding energy and mass to the system by burning fuel like the turbojet does. I suspect the required revs to get a good performance would be in the order of 150K or so.

Compared to an axial flow design that can achieve it's thrust within the constraints of standard motor RPM's you may want to reconsider your plan. Axial flow eliminates the super high RPM's and also it will eliminate the need for a power robbing and probably heavy build up gearbox.

If you're looking for new horizons to explore I can't help but wonder if a dual blade ducted E fan system using a lower pitched first blade powered by one motor and then backed up by a second motor that would input further energy to provide for a truly high speed exhaust speed might be the way to go. A single blade fan can only accelerate the air so much. And making the two motors run independently would avoid the need to finely tune the blade pitches that is needed when two stages are driven by the same shaft. With two motors in one axial unit it would just be a case of being close enough with the pitches to allow each fan to do it's best work just shy of stalling the blades.